1. Field of the Invention
The present invention relates to an apparatus and a method for evaluating a projection lens and, more particularly, to an apparatus and a method for evaluating the curvature of the image field of a projection lens.
2. Description of the Related Art
FIG. 11 illustrates a known projection-exposure apparatus used in the production process for LSIs. The light emitted from a light source 11, such as a mercury lamp, travels through a condenser lens 12 to strike a mask 13 which has a circuit pattern of a chip thereon. The light transmitted through the mask 13 travels through a projection lens 14 to strike a wafer 15 so that the circuit pattern of the mask 13 is projected on the wafer 15. The circuit pattern is thus projected and transferred.
To project a fine and minute circuit pattern with high precision in this projection-exposure apparatus, the imaging plane of the projection lens 14 must be very flat. If the projection lens has a lens forming error such as tilting or curvature of the image field, the best focus position is not achieved in some locations so that the projection cannot be made onto the entire image field with the best focus.
According to the conventional art, the curvature of the image field of the projection lens 14 is evaluated as follows. As the wafer 15 is successively moved a predetermined distance at time intervals along the optical path, the wafer 15 is sequentially exposed to test patterns 16a to 16g as shown in FIG. 12. After the wafer 15 is developed, i.e., after patterns 17a to 17g corresponding to the test patterns 16a to 16g are formed on the wafer 15 as shown in FIG. 13, the patterns 17a to 17g are observed with a microscope. The best shaped pattern is selected from the patterns 17a to 17g as the best focus pattern. The position of the wafer 15 when exposed to the test pattern corresponding to the best shaped pattern is determined as the best focus position. In FIG. 13, the pattern 17c is determined as the best shaped pattern. In this way, the best focus positions are determined with respect to many locations in a chip on the wafer 15 which is exposed to the circuit pattern on the mask 13. The distribution of the best focus positions in a chip is examined to evaluate the curvature of the image field of the projection lens 14.
As described above, the image field of the projection lens 14 is evaluated by exposing wafer 15 to the test patterns 16a to 16g, developing the wafer 15, observing the patterns 17a to 17g with an optical microscope, an electron microscope or the like to determine the best focus positions with respect to many locations in a chip. Thus, evaluation of a projection lens according to the conventional art requires a lot of steps and time.